Prodigiosin, Violacein, and Volatile Organic Compounds Produced by Widespread Cutaneous Bacteria of Amphibians Can Inhibit Two <italic>Batrachochytrium</italic> Fungal Pathogens.

Symbiotic bacteria can produce secondary metabolites and volatile compounds that contribute to amphibian skin defense. Some of these symbionts have been used as probiotics to treat or prevent the emerging disease chytridiomycosis. We examined 20 amphibian cutaneous bacteria for the production of prodigiosin or violacein, brightly colored defense compounds that pigment the bacteria and have characteristic spectroscopic properties making them readily detectable, and evaluated the antifungal activity of these compounds. We detected violacein from all six isolates of <italic>Janthinobacterium lividum</italic> on frogs from the USA, Switzerland, and on captive frogs originally from Panama. We detected prodigiosin from five isolates of <italic>Serratia plymuthica</italic> or <italic>S. marcescens</italic>, but not from four isolates of <italic>S. fonticola</italic> or <italic>S. liquefaciens</italic>. All <italic>J. lividum</italic> isolates produced violacein when visibly purple, while prodigiosin was only detected on visibly red <italic>Serratia</italic> isolates. When applied to cultures of chytrid fungi <italic>Batrachochytrium dendrobatidis</italic> (<italic>Bd</italic>) and <italic>B. salamandrivorans</italic> (<italic>Bsal</italic>), prodigiosin caused significant growth inhibition, with minimal inhibitory concentrations (MIC) of 10 and 50 μM, respectively. Violacein showed a MIC of 15 μM against both fungi and was slightly more active against <italic>Bsal</italic> than <italic>Bd</italic> at lower concentrations. Although neither violacein nor prodigiosin showed aerosol activity and is not considered a volatile organic compound (VOC), <italic>J. lividum</italic> and several <italic>Serratia</italic> isolates did produce antifungal VOCs. White <italic>Serratia</italic> isolates with undetectable prodigiosin levels could still inhibit <italic>Bd</italic> growth indicating additional antifungal compounds in their chemical arsenals. Similarly, <italic>J. lividum</italic> can produce antifungal compounds such as indole-3-carboxaldehyde in addition to violacein, and isolates are not always purple, or turn purple under certain growth conditions. When <italic>Serratia</italic> isolates were grown in the presence of cell-free supernatant (CFS) from the fungi, CFS from <italic>Bd</italic> inhibited growth of the prodigiosin-producing isolates, perhaps indicative of an evolutionary arms race; <italic>Bsal</italic> CFS did not inhibit bacterial growth. In contrast, growth of one <italic>J. lividum</italic> isolate was facilitated by CFS from both fungi. Isolates that grow and continue to produce antifungal compounds in the presence of pathogens may represent promising probiotics for amphibians infected or at risk of chytridiomycosis. In a global analysis, 89% of tested <italic>Serratia</italic> isolates and 82% of <italic>J. lividum</italic> isolates were capable of inhibiting <italic>Bd</italic> and these have been reported from anurans and caudates from five continents, indicating their widespread distribution and potential for host benefit. [ABSTRACT FROM AUTHOR]